Morphology parameters: Substructure identification in X-ray galaxy clusters

Context. In recent years multi-wavelength observations have shown the presence of substructures related to merging events in a large proportion of galaxy clusters. Clusters can be roughly grouped into two categories-relaxed and non-relaxed-and a proper characterisation of the dynamical state of these systems is crucial for both astrophysical and cosmological studies. Aims. In this paper we investigate the use of a number of morphological parameters (Gini, M₂₀, concentration, asymmetry, smoothness, ellipticity, and Gini of the second-order moment, G_M) introduced to automatically classify clusters as relaxed or dynamically disturbed systems. Methods. We apply our method to a sample of clusters at different redshifts extracted from the Chandra archive and investigate possible correlations between morphological parameters and other X-ray gas properties. Results. We conclude that a combination of the adopted parameters is a very useful tool for properly characterising the X-ray cluster morphology. According to our results, three parameters-Gini, M₂₀, and concentration-are very promising for identifying cluster mergers. The Gini coefficient is a particularly powerful tool, especially at high redshift, because it is independent of the choice of the position of the cluster centre. We find that high Gini (>0.65), high concentration (>1.55), and low M₂₀ (<-2.0) values are associated with relaxed clusters, while low Gini (<0.4), low concentration (<1.0), and high M₂₀ (>-1.4) characterise dynamically perturbed systems. We also estimate the X-ray cluster morphological parameters in the case of radio loud clusters. Since they are in excellent agreement with previous analyses we confirm that diffuse intracluster radio sources are associated with major mergers.